The embodiments herein generally relate to media registration in duplex printers. As explained in U.S. Patent Publication 2004/0036847, the complete disclosure of which is incorporated herein by reference, duplex printing can be conducted with a system that forms images on both sides of a sheet at a single transfer station. In some duplex printing systems, after a sheet has received a first image on a first side by passing the sheet through an image transfer station, the sheet is inverted. A second image is then formed on a second side of the sheet by passing the inverted sheet through the same transfer station.
Positioning an image on one side of a sheet in a manner that coincides with the position of the image on the other side of the sheet can be difficult. Registration of a first image on a first side of a sheet with a second image on a second side of a sheet is not always accurate because of one or more registration errors that offset the first image relative to the second image. For example, a page number printed on the bottom-center position of the first side of a two-sided, printed document should align exactly with the page number printed on the reverse side. The offset of the page number on the second side of a sheet with respect to the page number on the first side of the sheet is a registration error that is extremely undesirable, and considered unacceptable in various printing industries.
Registering two images on the front and back sides of a single sheet of media sheet can be difficult but is essential in industries such as the offset printing industry. In this industry, duplex sheets are sometimes produced having a number of pages, of what will ultimately be a single, multi-page document, aligned on the front and back of a single sheet of media sheet. To create such a multi-page document, a sheet of media sheet is printed with multiple images on the front and back side of a single composite sheet. The single composite sheet is subsequently folded and segmented into individual pages. Each of the images on a first side a sheet must therefore be registered with a corresponding image on a second side of the sheet before the sheet may be segmented into individual pages.
Specifically, the first image that appears on the first side of the sheet and the second image that appears on the second side of the sheet are positioned so that identical images printed on both sides of the sheet are coincident with each other. In other words, two identical images printed on both sides of a sheet of media sheet form mirror images of each other since each image is printed with no apparent offset from the other. Thus, an image on the front side of a sheet would appear to be in perfect or transparent registration with the corresponding image on the back side of the sheet.
The “show through” error that occurs when transparent registration is not achieved can be quantified by measuring of the displacement between two points, one on a first side of the sheet and one on a second side of the sheet, that are intended to be equidistant from a common sheet edge. This error is caused, at least in part, by the factors identified above. The portion of the error associated with media sheet shrinkage is often caused by fusing a printed image on the first side prior to printing of an image on the second side.
Additional background patents that are incorporated herein by reference include U.S. Patent Publication No. 20030146567 published Aug. 7, 2003; U.S. Pat. No. 4,971,304 by Lofthus, issued Nov. 20, 1990; U.S. Pat. No. 5,169,140 by Wenthe, Jr., issued Dec. 8, 1992; U.S. Pat. No. 5,219,159 by Malachowski et al, issued Jun. 15, 1993; U.S. Pat. No. 5,278,624 by Kamprath et al, issued Jan. 11, 1994; U.S. Pat. No. 5,794,176 by Milillo, issued Aug. 11, 1998; U.S. Pat. No. 6,137,989 by Quesnel, issued Oct. 24, 2000; U.S. Pat. No. 6,168,153 B1 by Richards et al, issued Jan. 2, 2001; and U.S. Pat. No. 6,533,268 B2 by Williams et al, issued Mar. 18, 2003.